In order to achieve a long-distance and large-capacity transmission system, techniques for transmitting optical signals using digital signal processing have been studied and developed. Optical transmitters using digital signal processing can generate optical signals in any desired modulation format. For example, generating optical signals using multilevel modulation formats (QPSK, 16QAM-256QAM and the like) increases the number of bits per symbol. In addition, digital signal processing can provide multicarrier transmission (OFDM, Nyquist WDM and the like). Also, optical receivers using digital signal processing can compensate for the waveform distortions of received optical signals. As an example, there is known a method of using digital signal processing to compensate for waveform distortions due to chromatic dispersion or the like.
Polarization multiplexing has been in practical use as another technique to achieve large-capacity transmission. In polarization multiplexing, two optical signals are transmitted using a pair of orthogonal polarizations (x-polarization and y-polarization). Therefore, polarization multiplexing transmission can double transmission capacity compared to single-polarization transmission. Generally, in polarization multiplexing transmission, it is preferable that optical signals in different polarization channels have the same optical power.
An optical transmission line, however, has polarization dependent loss (PDL). If polarization multiplexed optical signals are transmitted via a transmission line that has polarization dependent loss, the optical signals in an x-polarization channel and a y-polarization channel will suffer different losses. That is, in an optical transmission line that has polarization dependent loss, an optical power difference arises between the polarization channels. In this case, the characteristics of the optical signals (in particular, optical signal-to-noise ratio, OSNR) differ between the polarization channels.
Configurations taking this problem into consideration have been proposed. As one example, a configuration (e.g., Japanese Laid-Open Patent Publication No. 2009-89194) is proposed that varies the allocation of data (bits) to the polarization channels so that each bit of the channel to be transmitted by x-polarization and each bit of the channel to be transmitted by y-polarization in the case of normal allocation are switched around in a required ratio. As another example, a configuration (e.g., Japanese Laid-Open Patent Publication No. 2010-109705) is proposed that implements polarization scrambling of transmitted signal light by digital signal processing.
According to the above proposed configurations, even if there is polarization dependent loss, degradations in the polarization channels are equalized with respect to the time axis to reduce differences in average bit error rate between the polarization channels. These configurations, however, do not reduce the difference in OSNR between the polarizations and therefore cannot sufficiently improve transmission characteristics.
Meanwhile, using digital signal processing enables compensating for the waveform distortions of optical signals. Therefore, chromatic dispersion and/or polarization mode dispersion may be compensated for by digital signal processing. However, since polarization dependent loss causes degradation of OSNR, it is difficult to compensate for the degradation of transmission characteristics due to polarization dependent loss even if digital signal processing is used for an optical receiver.